1. Field of the Invention
The present invention relates to a betatron with a removable accelerator block, particularly for producing x-radiation in an x-ray inspection system.
2. Description of the Background Art
X-ray inspection systems are used, as is well-known, in the inspection of large-volume articles such as containers and motor vehicles for illegal contents such as weapons, explosives, or contraband goods. In so doing, x-radiation is produced and directed at the article. The x-radiation attenuated by the article is measured by a detector and analyzed by an evaluation unit. Therefore, a conclusion can be reached on the nature of the article. This type of x-ray inspection system is known, for example, from European Pat. No. EP 0 412 190 B1, which corresponds to U.S. Pat. No. 5,065,418.
Betatrons are used to generate x-radiation with the energy of more than 1 MeV needed for the inspection. These are circular accelerators in which electrons are held in an orbit by a magnetic field. A change in this magnetic field produces an electric field, which accelerates the electrons in their orbit. A stable nominal orbit radius is determined from the so-called Wideroe condition depending on the course of the magnetic field and its change with time. The accelerated electrons are guided onto a target, where upon impacting they produce Bremsstrahlung whose spectrum depends, inter alia, on the energy of the electrons.
A betatron disclosed in German Patent Application No. DE 23 57 126 A1 includes a two-part inner yoke, in which the front sides of both inner yoke parts face each other spaced apart. A magnetic field is produced in the inner yoke by means of two main field coils. An outer yoke connects the two inner yoke part ends distant from one another and closes the magnetic circuit.
An evacuated betatron tube, in which the electrons to be accelerated circulate, is arranged between the front sides of the two inner yoke parts. The front sides of the inner yoke parts are formed in such a way that the magnetic field produced by the main field coil forces the electrons into a circular orbit and moreover focuses them onto the plane in which this orbit lies. To control the magnetic flux, it is prior in the art to arrange a ferromagnetic insert between the front sides of the inner yoke parts within the betatron tube.
Because of the produced x-radiation, betatrons are provided with lead shielding, which enables emission of the radiation only at defined places. In prior-art betatrons, for maintenance of the accelerator block, part of the lead shielding must be loosened and removed. Then, the inner part, including the accelerator block and the outer yoke, is lifted out. This has the disadvantage that in each case large masses must be moved and devices suitable for this are needed.